JPH0352903A - Production of water-absorptive resin with little water-solubles - Google Patents

Abstract

PURPOSE:To obtain the title resin useful for hygienic articles, agricultural and horticultural articles, etc., by aqueous solution polymerization of a (meth)acrylic alkali metal salt in the presence of water-absorptive resin powder to effect crosslinking. CONSTITUTION:The objective resin can be obtained by aqueous solution polymerization of a monomer aqueous solution containing as the main monomer a (meth)acrylic alkali metal salt in the presence of water-absorptive resin powder [pref. of a crosslinked polymer from (meth)acrylic alkali metal salt as the main monomer] plus pref. a crosslinkable compound to effect crosslinking.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a new method for producing a water-absorbing resin with a low water-soluble content. (Prior art) Water-absorbing resins are used in sanitary products such as sanitary products, diapers, and disposable rags, and in agricultural and gardening products such as water retention agents, as well as in coagulation of sludge, prevention of condensation on building materials, dehydration of oils, etc. It is also used for purposes such as Examples of this type of water-absorbent resin include carboxymethylcellulose crosslinked products, polyoxyethylene crosslinked products, starch-acrylonitrile graft copolymers hydrolysates, starch-(meth)acrylic acid graft copolymers,
(Meth)acrylate polymer crosslinked products, (meth)acrylate copolymer crosslinked products, etc. are known. Among these,
Carboxymethylcellulose crosslinked products and polyoxyethylene crosslinked products have not yet been found to have satisfactory water absorption and water retention abilities.

Therefore, (meth)acrylate polymer crosslinked product, (meth)
The cross-linked acrylate copolymer satisfies water absorption capacity, water retention capacity, quality stability, etc. Conventionally, known methods for producing crosslinked (meth)acrylate-based polymers include aqueous solution polymerization, reverse-phase emulsion polymerization, and reverse-phase turbid polymerization. Among these methods, particularly in the case of aqueous solution polymerization, the water-absorbent resin obtained conventionally contains a considerable amount of water-soluble resin, which is a water-soluble component, and therefore, when used in the sanitary products mentioned above, However, due to the water-soluble content, it has the disadvantage of causing discomfort such as a sticky feeling when used.
One way to reduce the water-soluble content in a water-absorbing resin is to increase the amount of cross-linking monomer used during polymerization, but such methods significantly reduce the water-absorbing ability of the resulting water-absorbing resin. There is a tendency to decrease. (Problems to be Solved by the Invention) Based on the above-mentioned circumstances, it is possible to improve the water-soluble content of the crosslinked (meth)acrylate polymer without degrading its original properties as a water-absorbing resin. There is a strong need in the industry for the development of a method for producing a vine water-absorbing resin that reduces the amount of water.

It is therefore an object of the present invention to produce a water-absorbing resin that has excellent properties such as water-absorbing ability of a cross-linked (meth)acrylate-based polymer and has an extremely low water-soluble content by aqueous solution polymerization. The aim is to provide the technology for production. (Means for Solving the Problems) In view of the above-mentioned circumstances, the present inventors, in order to achieve the above-mentioned object,
In particular, we have conducted intensive research on various polymerization conditions for aqueous solution polymerization of (meth)acrylate-based polymers and fruit fruits.
As a result, it was surprisingly discovered that this objective could be achieved by making water-absorbing resin powder present during the polymerization. The present invention was completed based on this knowledge.

That is, the present invention provides a method for producing a water-absorbing resin by polymerizing an aqueous monomer solution containing an alkali metal acrylate and/or an alkali metal methacrylate as a main monomer. This invention relates to a method for producing a water-absorbing resin characterized by polymerization and crosslinking in the presence of fat powder. In the present invention, the main monomer unit constituting the target water-absorbent resin is an alkali metal salt of (meth)acrylic acid. Here, the alkali metal salt refers to sodium salt or potassium salt. These alkali metal compounds are obtained by neutralizing (meth)acrylic acid with sodium hydroxide, potassium hydroxide, etc., and these are not necessarily completely neutralized products, but partially neutralized products. There is no problem even if the degree of neutralization is 50% to 0%, more preferably 60% to 85%. If it is less than 501, it is not preferable because the water absorbing ability of the resulting water absorbent resin tends to decrease. In the present invention, the main monomer is an alkali metal salt of (meth)acrylic acid as described above, but if necessary, a divinyl compound or a polyepoxy compound may be used in combination as a crosslinking compound. Preferred divinyl compounds include, for example, divinylbenzene.
N,N'-methylenebisacrylamide, N,N'-methylenebismethacrylamide, polyethylene glycol diacrylate, polybrobylene glycol diacrylate and the like can be used alone or in combination. On the other hand, examples of the polyepoxy compound include diglycidyl ethers and triglycidyl ethers having an epoxy group of 2M or more. Examples of diglycidyl ethers include ethylene glycol diglycidyl ether. Polyethylene glycol diglycidyl ether, brobylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether,! ．． 6-hexanediol diglycidyl ether, glycerin-1
．． Examples include 3-diglycidyl ether and neopentyl glycol diglycidyl ether. Among these, preferred are ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, and polypropylene glycol diglycidyl ether. Here, the degree of polymerization of the alkylene oxide in the polyalkylene glycol diglycidyl ether is preferably about 2 to 50. Examples of triglycidyl ethers include trimethylolbroban triglycidyl ether,
Examples of polyglycidyl ethers having higher functional groups include diglycerin tetraglycidyl ether, triglycerin bentaglycidyl ether, sorbitol tetraglycidyl ether, sorbitol bentaglycidyl ether, and sorbitol hexaglycidyl. Examples include ethers and pentaerythritol tetraglycidyl ethers. Preferred among these include trimethylolpropane triglycidyl ether, glycerin triglycidyl ether, and polyglycerin polyglycidyl ether. Furthermore, examples of polyepoxy compounds include shrimp chlorohydrin modified products of polyamide-boreamine condensates. These divinyl compounds and polyepoxy compounds may be used alone or in combination. The amount of the divinyl compound or polyepoxy compound to be used is determined appropriately taking into consideration the water absorption capacity of the resulting water absorbent resin, gel strength, stability of gel strength over time, etc., but usually the total amount of these is calculated based on the total amount of the water absorbent resin. It is recommended that the weight i be approximately 5.0 weight i or less, preferably 1.0 weight i or less. By the way, in the case where the conventional aqueous solution polymerization method is adopted, it is necessary to use the crosslinkable monomer in an amount of at least 0.001 weight i. Contains a considerable amount of soluble matter. On the other hand, the water-absorbing resin obtained by the method of the present invention does not need to contain the crosslinking monomer as an essential component, and the amount of the crosslinking monomer used is o. It has the advantage of being able to obtain a water-absorbing resin with extremely low soluble content even if the weight is less than about i. In addition, if it exceeds 5.0 weight 1, it is not preferable because the water absorption capacity tends to decrease. Furthermore, in addition to the alkali metal (meth)acrylic acid salt, which is the essential monomer, other monomers may be used in combination, if necessary. Such other monomers are preferably acrylic or methacrylic monomers, and specific examples include acrylamide, acrylamide-2-methylpropanesulfonate, lower acrylic esters, and lower methacrylic esters. The amount of the monomer to be used is preferably about 20 parts by weight or less of the total monomers, taking into consideration the water absorbing capacity, water holding capacity, gel strength, etc. of the resulting water absorbent resin. Additionally, if necessary, a surfactant can be used. Specific examples include polyoxyethylene alkyl ether, polyoxyethylene alkyl phenyl ether, polyoxyethylene sorbitan fatty acid ester, higher alcohol sulfate salt, alkylbenzene sulfonate, polyoxyethylene sulfate salt, and the like. The amount used is 0.0% based on the total monomer. Ol~
5 weight i. The polymerization initiators used in the present invention are not particularly limited, and include various known ones, such as persulfates such as ammonium persulfate and potassium persulfate, redox systems of persulfates and sulfites, various water-soluble azo compounds, etc. can be used. Specific examples of water-soluble azo compounds include azobisisobutyronitrile, 4-t-butylazo-4-cyanovaleric acid, 4,4゛-azobis(4-cyanovaleric acid 1,2.2
゜-azobis{2-amidinobroban} dihydrochloride, 2.
Examples include 2°-azobis(N,N'-dimethyleneisobutyramidine) dihydrochloride and 2.2°-azobis(N,N'-dimethyleneisobutyramidine). In the present invention, aqueous polymerization can be initiated not only by heating but also by ultraviolet irradiation. In the case of ultraviolet polymerization, water-soluble azo compounds are preferred among the initiators, and among them, 2,2゛-azobis(2-amidinobroban) dihydrochloride, 2.2゜-azobistN,N'-dimethyleneisobutyl Amidine) dihydrochloride, 2,2°-azobis(N,N'-dimethylene isoptylamidine), and the like are preferred. In this case, diacetyl, benzoin, benzyl, anthraquinone, acetophenone, diphenyl disulfide, penzophenone, and various derivatives thereof as photopolymerization initiators generally used in photopolymerization are not suitable. This is because when such an initiator is used, polymerization is not completed and a considerable amount of unreacted monomer remains, resulting in the resulting water-absorbent resin having a sticky feel when water is absorbed.

In the method of the present invention, it is essential to have water-absorbing resin powder present during polymerization, and only then can the above-mentioned object of the present invention be achieved. There are no particular restrictions on how the water-absorbing resin powder can be present, and it is usually sufficient to allow the resin powder to be dispersed somewhat uniformly within the reaction system. There are no particular restrictions on the water-absorbent resin powder used in combination during polymerization, and any known variety can be used, but (meth)acrylic acid is usually used in view of the effect of reducing the water-soluble content in the resulting water-absorbent resin. A crosslinked polymer is more preferred. Examples of (meth)acrylic acid-based polymers include polyacrylic U salt cross-linked products, polymethacrylate cross-linked products, polyacrylic acid-methacrylate cross-linked products, and hydrolyzed products of acrylonitrile graft copolymer , starch one (meta)
Examples include acrylic acid graft copolymers. There are no particular restrictions on the particle size of the resin powder, but it is usually about 1,000 microns or less, preferably about 300 microns or less. The amount of the water-absorbing resin powder to be used is not particularly limited, but it is usually about 0.5 to 20 parts by weight of the total monomers used in the aqueous solution polymerization. If it is too less than 0.5 weight i, the effect of its addition will be insufficient, and if it is used in excess of 20 weight i, there is no point in using it in excess. The production method of the present invention is substantially similar to conventionally known methods, except for the use of a water-absorbing resin during polymerization. As such a conventionally known method, for example, Japanese Patent Application Laid-Open No. 159-1989
823, JP 55-84304, JP 55-108407, JP 55-133413
No. 1, JP-A-56-36504, JP-A-56-
91837, JP 58-71907, JP 63-43912, JP 63-31751
I will give examples such as Publication No. 9. More specifically, the manufacturing method of the present invention is carried out as follows. First of all, a predetermined amount of an alkali metal acrylate salt and/or an alkali metal acrylate salt and other monomers, cross-linkable substances, surfactants, etc. used as necessary are added to water. to prepare an aqueous monomer solution.

The concentration of all the monomers in the monomer aqueous solution is usually 30 to 80 wt i, preferably 40 wt when polymerizing by heating.
It is best to set the weight to ~75. In the case of ultraviolet irradiation, it is usually 25 to 65% by weight, preferably 30 to 60% by weight.
It is better to Here, if the monomer concentration is less than the lower limit, the degree of polymerization of the resulting water-absorbent resin tends to decrease, while if it exceeds the upper limit, the reaction temperature during the reaction becomes too high, resulting in a decrease in the water-absorbent resin obtained. The resin tends to become porous, which tends to reduce water retention. Next, the polymerization initiator is stirred and mixed into the monomer aqueous solution to dissolve the polymerization initiator. The amount of polymerization initiator used is
Although not particularly limited, it is usually 0.0% based on all monomers.
01-5.0 weight about 1, preferably o. oi-t
．． o It is appropriate to set the weight to 1. Next, after charging this mixed solution into a suitable reaction vessel, the reaction system is heated or irradiated with ultraviolet rays to initiate polymerization and crosslinking reactions. However, if the monomer concentration during polymerization is about 60% by weight or more, no heating operation is required during polymerization,
The reaction can be completed only by the heat of polymerization. In the polymerization and crosslinking reactions of the present invention, the shape of the reaction container is not limited, but it is preferable to use an endless belt or an open container with a large surface area for convenience such as drying the resulting water absorbent resin and workability in the pulverization process. ．． There are also no particular restrictions on the thickness of the reaction solution when supplied, but it is usually 5 cm, especially in the case of ultraviolet irradiation, in order to allow sufficient ultraviolet rays to pass through.
It is best to keep it below. In addition, the amount of light for ultraviolet cotton is not particularly limited, but is usually 5 to 200 (lw).
Joul/ca+" is preferable. If it is less than this range, polymerization and crosslinking tend to be insufficient. The preferable amount of light is about 50 to 10 to 1 Joul/c. Also, ultraviolet irradiation. As the light source used for this process, conventionally known light sources such as incandescent lamps, halogen lamps, fluorescent lamps, xenon lamps, sodium lamps, mercury lamps, metal halide lamps, etc. can be used as they are, depending on the reaction conditions such as the liquid thickness, etc. be selected and used as appropriate. Thus, by heating the reaction system or irradiating it with ultraviolet light,
The reaction starts immediately, and the polymerization and crosslinking reactions usually proceed at about 70 to 100°C. The ultraviolet irradiation time during ultraviolet polymerization is appropriately determined so as to achieve the above-mentioned light intensity. For example, if an endless belt is used, the reaction will normally start after a short period of several seconds to several minutes at the irradiated area under the above-mentioned conditions. is completed. The temperature for preparing the mixture of the monomer aqueous solution and the photopolymerization initiator to be subjected to the irradiation reaction is preferably about 0 to 40°C, preferably 10 to 25°C. If the temperature is less than 0℃, the mixture may solidify, and if it exceeds 40℃, the reaction system temperature will become too high in the subsequent polymerization reaction, resulting in a porous water-absorbing resin with a low maintenance level. be. The water absorbent resin thus obtained usually has a water content of 35 to 75 parts by weight! 1, and its appearance is that of a transparent rubber-like elastic body. Therefore, if necessary depending on the application,
Subsequently, the polymer is dried and pulverized to form a powder,
Granules and vines. For these steps, it is sufficient to apply known methods as they are, and there is no need to use special operations or equipment. For example, a 12m hot air dryer, an infrared dryer, a fluidized bed dryer, etc. can be used as a drying device, and the drying temperature is usually 70~
The temperature should be around 200℃. The obtained dry water-absorbent resin can be pulverized to a desired particle size using, for example, a vibratory pulverizer or an LFR percussion pulverizer. In addition, in the above polymerization reaction, when the polymerization is carried out by heating at a high monomer concentration, a water-absorbing resin with a water content of about 10 wt. Direct pulverization becomes possible. The water-absorbing resin obtained by the method of the present invention, which requires the use of water-absorbing resin powder in combination during polymerization, has a higher water-absorbing resin than the water-absorbing resin obtained by exploring the various conventional aqueous solution polymerization methods described above. Despite its characteristic of having an extremely low content of dissolved components, the water-absorbing resin's inherent water-absorbing ability and various other properties have hardly deteriorated. In addition, among the conventional techniques, Japanese Patent Application Laid-Open No. 63-43912 or Japanese Patent Application Laid-Open No. 63-317
The water-absorbing resin obtained by the method described in Publication No. 519 has a considerably lower content of water-soluble components than those obtained by other known techniques, but in any case, the water-absorbing resin obtained by the method described in Publication No. It goes without saying that if the method of the present invention is applied, the water-soluble content can be further reduced. The size, shape, etc. of the water-absorbing resin obtained by the method of the present invention are not particularly limited and can be appropriately selected depending on the intended use. For example, when used as a sanitary material, it is usually in the form of particles, with approximately 100 to
Those having a particle size of 1000 μm are preferable. The water-absorbing resin obtained by the method of the present invention can be used for the various purposes mentioned above, and is particularly useful for sanitary products such as sanitary products and disposable diapers. The water-absorbing resin may be used alone or in combination with an inorganic powder such as silicon dioxide powder or titanium oxide powder, or an organic filler such as rubber, depending on the purpose.
Incidentally, examples of the silicon dioxide powder include colloidal silica, white carbon, and ultrafine particulate silica. Furthermore, it is also possible to post-modify the water-absorbing resin pulverized product obtained by the above method, if necessary. For example, carboxylate contained in a water-absorbing resin is reacted with a known crosslinking agent such as a polyepoxy compound typified by a water-soluble diglycidyl ether compound, an aldehyde compound, or a polyvalent metal salt, and the water-absorbing resin is pulverized. It is also possible to modify the surface of objects, and the modified product can also be used for the same purposes as above. (Examples) Hereinafter, the present invention will be explained in more detail with reference to reference examples, working examples, and comparative examples, but it goes without saying that the present invention is not limited only to these examples. Reference Example 1 As a water-absorbing resin powder, a polyacrylic acid-based crosslinked product (trade name "Arasorb S-100J", manufactured by Arakawa Chemical Industry Co., Ltd.) was sieved through an 80-mesh wire mesh (hereinafter referred to as
This is called water-absorbing resin powder a), and its water absorption capacity is 55g.
/g. Reference Example 2 As a water-absorbing resin powder, a polyacrylic acid-based resin powder (trade name: "Aqualic CA J", manufactured by Nippon Shokubai Chemical Co., Ltd.) was sieved through an 80-mesh wire mesh (hereinafter referred to as (referred to as water absorbent resin powder b), its water absorption capacity is 44
g/g. Reference Example 3 As a water-absorbing resin powder, a Denbun-acrylic acid-based graft copolymer (trade name "Sunwet IM-1500J") was used.
, manufactured by Sanyo Chemical Industries, Ltd.) through an 80-mesh wire mesh (hereinafter referred to as water-absorbing resin powder C), and its water absorption capacity is 47 g/g.

Example 1 328 g of acrylic acid and 328 g of acrylic acid were cooled with water. Bisui 543. 2g
After neutralizing by adding 136.55 g of sodium hydroxide (equivalent to 75 mol i to acrylic acid), N, N'
- 0.032 g of methylenebisacrylamide (0.008 weight 1 relative to total monomers) and 2.2゛-
Azobis(N.N'-dimethyleneisobutyramidine) dihydrochloride 0.403g (0.1% based on total monomers)
Weight 1) was added and dissolved, and nitrogen gas was blown in to drive out the dissolved oxygen.
A prepared solution of 1% was obtained. 20.16 g of water absorbent resin a (5 weight 1 for all monomers) was added to the prepared liquid and stirred to disperse the water absorbent resin powder. Then, 176.7 g was taken and the inner diameter was 150-■
In addition to a glass petri dish with a cleanliness of 25 seconds, a conveyor-type ultraviolet device (manufactured by Eye Graphic Co., Ltd., high-pressure mercury lamp 2κwXZ lamp, 8017 cab, emission length 250 ms) was installed.
) at a belt speed of 10.8 seconds at a belt speed of 10.8 seconds to obtain a light intensity of 900 joui/cm" to obtain a hydrous gel crosslinked polymer with rubber elasticity and a thickness of 9 m. Next, cut this gel-like crosslinked polymer into two parts using a cutter.
This was cut into ~5 pieces, dried in a hot air dryer at 140°C for 2 hours, pulverized in a pulverizer, and further dried at 130°C for 30 minutes to obtain water absorbent resin powder A.

Example 2 In Example 1, the amount of water absorbent resin powder a added was 40.
Water-absorbing resin powder B was obtained by polymerizing, drying, and pulverizing in the same manner except that 33 g (IO weight % based on the total heptamer) was used. Example 3 The same procedure as in Example 1 was carried out except that water-absorbing resin powder b was used instead of water-absorbing resin powder a, and the amount added was changed to 40.33 g (10% by weight based on the total monomers). Polymerization, drying, and pulverization were performed to obtain water absorbent resin powder C. Example 4 Same procedure as in Example 1 except that water-absorbent resin powder C was used instead of water-absorbent resin powder a, and the amount added was changed to 40.33 g (lO weight i relative to all monomers). Polymerization, drying, and pulverization were performed to obtain water-absorbing resin powder D. Example 5 30 g of acrylic acid was mixed with 9.9 g of water. In addition to 24 g, 20.6 g of potassium hydroxide with a purity of 85% (equivalent to 75 mol 1 based on acrylic acid) was added to neutralize it, and then N,N
0.009 g of ゛-methylenebisacrylamide (0.021 weight i based on all heptamers) was added to prepare an aqueous potassium acrylate solution with a monomer concentration of 70 weight i.
4.19 g of water-absorbing resin powder a (IO weight i relative to all monomers) was added to the prepared liquid and stirred to disperse the resin powder, followed by keeping the temperature at 70°C. To this, 1 g of water and 0.0 g of 2.2'-azobis(2-amidinobroban) dihydrochloride. 2
A solution containing 0.08 g was added and immediately spread on the bottom of a cylindrical reactor having an inner diameter of about 10 cm.

Polymerization started after a few seconds and was completed within 1 minute, resulting in a crosslinked polymer that foamed and dried due to the heat of polymerization. This was crushed to obtain a crosslinked polymer powder. Next, this crosslinked polymer powder was dried in a hot air dryer at 130° C. for 30 minutes to obtain water absorbent resin powder E.

Example 6 350 g of acrylic acid and 504 g of water under water cooling. 62g
After neutralizing by adding 145.71 g of sodium hydroxide (equivalent to 75 mol 2 to acrylic acid), N, N'
- After adding and dissolving 0.086 g of methylenebisacrylamide (0.02 weight i based on all monomers) and 2.15 g of sorbitan fatty acid ester (manufactured by Kao Atlas Co., Ltd., trade name "Suban 20"), M gas was added. The dissolved oxygen was blown out and the liquid temperature was brought to 40°C. 43.01 g of water-absorbent resin powder a (IO weight i based on all monomers) was added to the prepared solution, stirred, and after the water-absorbent resin powder was separated, 0.0 g of ammonium persulfate was added. 15g and sub-Et@
0.05 g of sodium hydrogen was added and dissolved. This solution was lined with tetrafluoroethylene.150mm
I put it in a corner container. Polymerization proceeded slowly, and the temperature of the reaction system reached 55 to 60°C 2 hours after the start of polymerization. Seven hours after the start of polymerization, the gel-like crosslinked polymer content was taken out. Next, the polymer was cut into pieces of about 2 to 5+ ms with a cutter, dried in a hot air dryer at 140°C for 2 hours, and then ground with a grinder to obtain water absorbent resin powder F.

Example 7 In Example 1, N,N'-methylenebisacrylamide was not added, and the addition period of water absorbent resin powder a was changed to 40
．． Water-absorbing resin powder G was obtained by polymerizing, drying, turning into powder, polymerizing, drying, and pulverizing in the same manner except that 33 g (lO weight % based on the total monomers) was used. Comparative Example 1 A water-absorbing resin powder H was obtained by polymerizing, drying, and pulverizing in the same manner as in Example 1, except that water-absorbing resin powder a was not added. Comparative Example 2 In Example 1, N. Polymerization and drying were carried out in the same manner except that the amount of N'-methylenebisacrylamide added was increased to 0.121 g (0.03 weight i based on the total heptanomer) and water-absorbing resin dressing a was not added. , and pulverized to obtain water-absorbent resin powder I. Comparative Example 3 A water-absorbing resin powder J was obtained by polymerizing, drying and pulverizing in the same manner as in Example 5, except that water-absorbing resin powder a was not added. Comparison N4 Water-absorbing resin powder K was obtained by polymerizing, drying, and pulverizing in the same manner as in Example 6, except that water-absorbing resin powder a was not added. Table 1 shows the measurement results of the water absorbing capacity and water soluble content of the water absorbent resin powders (A-K) obtained in the various Examples and Comparative Examples described above. In addition, water absorption capacity refers to the amount of physiological saline absorbed per 1 g of water-absorbing resin powder. (Measurement of water soluble content) Pour 300 ml of distilled water into a 500 ml Erlenmeyer flask,
Accurately weigh 0.2 ± 0.02 g of water-absorbent resin powder, add to this while stirring, and leave in a dark place for 24 hours. Then, filter under reduced pressure using Millipore glass fiber filter paper.
Place the filtrate in the dryer at 105°C! Heating for 8 hours, measuring the heating residue, and calculating the water soluble content using the following formula: Water soluble content = (heating residue X tool/sample weight Table 1 (Effects of the invention) For example, despite using an aqueous solution polymerization method similar to conventional methods, the remarkable effect is that it is possible to easily provide a water-absorbing resin that has extremely low water-soluble content and has all the excellent properties inherent to water-absorbing resins. Arakawa Chemical Industry Co., Ltd.

Claims (4)

[Claims] (1) In a method for producing a water absorbent resin by polymerizing an aqueous monomer solution containing an alkali metal acrylate and/or an alkali metal methacrylate as a main monomer, in the presence of a water absorbent resin powder. A method for producing a water absorbent resin characterized by polymerization and crosslinking. (2) The production method according to claim 1, wherein a crosslinkable compound is contained during the aqueous solution polymerization. (3) The production method according to claim 1 or 2, wherein the water-absorbing resin powder is a crosslinked polymer containing an alkali metal acrylate and/or an alkali metal methacrylate as a main monomer. (4) Water-absorbing resin powder is 0.5 to 20% of the total monomer
The manufacturing method according to claim 1 or 2, wherein the content is % by weight.